All "significant" powers handed to Europe will have to be put to a referendum, the minister for Europe said as he sought to calm resurgent Tory Eurosceptism today.

David Lidington was attempting to assure wavering Conservative MPs that the EU referendum bill would mean a national poll on almost all significant transfers of powers to Brussels.

The government insisted there would be referendums on treaty changes and any substantial push from Brussels to usurp UK sovereignty.

As the bill returned to the Commons for its committee stage, it threatened to provoke the most serious rebellion of Tory backbenchers since the formation of the coalition.

Well-known Eurosceptics such as Bill Cash had warned they felt the legislation included too large a loophole for them to support it.

They said it allowed ministers to judge cases in which European treaty changes would need a referendum and feared such a loophole could allow ministers to agree to a change from Brussels and circumvent parliament.

Backbenchers were further concerned when it transpired that such a ministerial decision would be subject to judicial review, meaning the courts could end up sitting in judgment over parliament to decide which law changes would require a referendum.

Over the last fortnight, Tory MPs have grown increasingly worried that the coalition government is bowing too much to Lib Dem concerns. Some feel the spirit of the legislation waters down a commitment made by David Cameron in 2009 that any and all transfers of power would have to go to a referendum.

Lidington told BBC Radio 4's Today programme the government would have the "smallest possible margin" to avoid its pledge for a referendum on significant EU treaty changes.

He claimed the bill would uphold the sovereignty of parliament and that any change in EU "competencies" in future would by law have to go to a referendum. Any decisions by ministers would be contained in a bill and be subject to a vote, thereby allowing MPs to have their say, he said.

Lidington acknowledged some colleagues could not be persuaded, but claimed some Eurosceptics, including Lord Leach and Martin Howe did support the government on the current proposal.